The present invention relates to a motor and electric pump that use a consequent pole type rotor.
Japanese Laid-Open Patent Publication No. 2010-263763 describes a motor using a so-called consequent pole type rotor. The motor includes a stator having twelve slots and a rotor having eight magnetic poles. The rotor includes a rotor core having four salient poles and four magnets (neodymium magnets) embedded in the rotor core in the circumferential direction of the rotor core. The four magnets function as one of the magnetic poles. The four salient poles are arranged between adjacent ones of the four magnets and function as the other one of the magnetic poles. A motor using such a consequent pole type rotor or a driving device for an electric pump or the like using such a motor obtains higher output with a smaller and lighter structure and reduces costs.
Another type of motor employs a sensorless driving technique. The motor controls the drive current supplied to a stator based on a waveform of an induced voltage, that is, a control signal having the waveform of the induced voltage, without using a rotation sensor. However, in a motor using the consequent pole type rotor that embeds magnets in the rotor core, stable induced voltage cannot be obtained when the rotor starts to rotate. That is, a certain period of time is necessary until the induced voltage becomes stable. Thus, time is required before starting the execution of a control based on the induced voltage.
Further, in a motor that uses a consequent pole type rotor, the waveform of the induced voltage is irregular as compared with a normal motor that does not include salient poles and includes eight magnets forming eight magnetic poles. Thus, it is difficult to employ the sensorless driving technique in a motor that employs a consequent pole type rotor.
More specifically, in a normal motor, the distortion rate with respect to a sine wave is 1.3% and small in a waveform of the induced voltage for each phase. This allows for satisfactory employment of the sensorless driving technique. In contrast, FIG. 6 shows the waveform of the induced voltage for each phase (U phase, V phase, and W phase) of a motor that uses a consequent pole type rotor in which the electrical angle is equal (for example, 27° in mechanical angle) between the two ends of one magnetic pole (magnetic pole of the magnet) and the other magnetic pole (magnetic pole of the salient pole). In such a motor, as shown in FIG. 6, the distortion rate with respect to a sine wave is 21.1%. Further, in a Y-connection motor using a similar consequent pole type rotor, the waveform of the induced voltage between terminals (U-V, V-W, and W-U) becomes irregular as shown in FIG. 7. In a delta-connection motor using a similar consequent pole type rotor, the waveform of the induced voltage between terminals (U-V, V-W, and W-U) becomes irregular as shown in FIG. 8. It is considered that the waveforms become irregular as shown in FIGS. 7 and 8 because the magnetic poles of salient poles do not generate a forcible magnetic pole flow like the magnetic poles of magnets.
Further, in the waveforms of FIGS. 6 to 8, for example, the peaks are deviated from normal positions. More specifically, the peak interval is approximately 110° where it should be 90°. In addition, the waveform is asymmetric relative to each peak. It is difficult to use induced voltage having such an irregular waveform as a control signal. Thus, it is difficult to employ the sensorless driving technique for the control of a consequent pole type rotor.